1. Field of the Invention
The present invention relates to an apparatus for recording and reading out magnetooptic information and applied as a recording and reading out apparatus of a magnetooptic disk, a magnetooptic card, a magnetooptic tape, etc.
2. Description of the Related Art
For example, the following five literatures are disclosed as a proposed technique about an apparatus for recording and reading out magnetooptic information as an optical system for detecting a magnetooptic signal.
(1) Trikepps pp. 174-183 in Chapter 5 of "Magnetooptic disk" written by Nagao under the supervision of Imamura in 1986;
(2) Research meeting report of quantum electronics of Electronic communication society of Japan, OQE86-177;
(3) M. Aoki, et.al. "TE/TM mode splitter in a slab waveguide with a tapered transition", MOC' 91 Technical digest E3 60-63;
(4) Japanese Patent Application Laying Open (KOKAI) No. 57-205840;
(5) Japanese Patent Publication (KOKOKU) No. 2-37611;
A general magnetooptic signal detecting optical system for a magnetooptic disk shown in the above first literature is constructed by a beam splitter, an analyzer and a photodetector. The beam splitter is arranged on an optical path of disk-reflecting light to reflect light reflected on the magnetooptic disk. The analyzer is arranged on an optical path of light reflected on the beam splitter to rotate a plane of polarization of this reflected light. The photodetector detects the reflected light transmitted through this analyzer and reads out a magnetooptic reading out signal. This magnetooptic reading out signal is called a magnetooptic signal in the following description.
In the following description, reference numeral I.sub.0 designates an intensity of light incident to the analyzer. .+-..theta.k designates an angle of rotation of the polarizing plane provided by the magnetooptic disk. .theta.a designates an angle of rotation of the analyzer with respect to an extinction axis thereof. .eta.d designates a sensitivity of the photodetector. Reference numeral M designates an internal electric current gain as a multiplication factor of the photodetector. Reference numeral S designates a signal component of an output electric current of the photodetector. In this case, the signal component S is provided as follows. ##EQU1##
Here, .theta.k&lt;&lt;1 is set.
In detection of the magnetooptic signal, the angle .theta.k of rotation of a polarizing plane caused by magnetic Kerr effects is small so that the magnetooptic signal has a small amplitude. Therefore, a magnetooptic differential detecting method is used to increase an S/N ratio in the signal detection.
A magnetooptic signal detecting optical system for a magnetooptic disk using this magnetooptic differential detecting method is constructed by first and second beam splitters, first and second analyzers, first and second photodetectors and a differential amplifier.
The first beam splitter is arranged on an optical path of light reflected on the magnetooptic disk such that one portion of this disk-reflecting light is transmitted through the first beam splitter to a servo error signal detecting system and the other portion of the disk-reflecting light is reflected on the first beam splitter. The second beam splitter is arranged on an optical path of light reflected on the first beam splitter such that one portion of this reflected light is reflected on the second beam splitter and the other portion of this reflected light is transmitted through the second beam splitter. The first analyzer is arranged on an optical path of light transmitted through the second beam splitter. The second analyzer is arranged on an optical path of light reflected on the second beam splitter.
The first photodetector is arranged on an optical path of light transmitted through the first analyzer to photoelectrically convert this light. The second photodetector is arranged on an optical path of light transmitted through the second analyzer to photoelectrically convert this light. The differential amplifier receives a first signal from the first photodetector and a second signal from the second photodetector. The differential amplifier is electrically connected to each of the first and second photodetectors so as to differentially amplify these first and second signals and output a magnetooptic reading out signal. The first and second analyzers are arranged such that extinction axes of the first and second analyzers are perpendicular to each other.
In this case, similar to the above formula (1), a signal component S.sub.1 of the first photodetector is provided as follows. ##EQU2##
A signal component S.sub.2 of the second photodetector is provided as follows with respect to light transmitted through each of the analyzers having the perpendicular extinction axes. ##EQU3##
A magnetooptic signal is provided as follows by a difference between the signal components S.sub.1 and S.sub.2 as outputs of the first and second photodetectors. ##EQU4##
As can be seen from the above formula (4), an amplitude of the magnetooptic signal in the differential signal detection is doubled in comparison with that in the single signal detection provided by the first formula (1). In a differential detecting system, a signal component based on a change in light intensity does not depend on polarization with respect to laser noises, disk noises and medium noises, and is outputted as the same phase component in a magnetooptic signal detecting system using the magnetooptic differential detecting method. Accordingly, signal components based on the change in light intensity are cancelled by calculating a difference therebetween.
An actual magnetooptic signal detecting optical system is constructed by a beam splitter, a 1/2 wavelength plate, a polarizing beam splitter, first and second photodetectors and a differential amplifier. The beam splitter is arranged on an optical path of light reflected on a magnetooptic disk such that one portion of this reflected light is reflected on the beam splitter. The 1/2 wavelength plate is arranged on an optical path of polarized light reflected on the beam splitter such that .theta.a is equal to 45.degree. by converting or transforming a plane of vibration of this polarized light. The polarizing beam splitter is arranged on an optical path of light transmitted through the 1/2 wavelength plate such that this transmitted light is separated into two polarizing light components.
The first photodetector is arranged on an optical path of one of the above two polarizing light components separated by the polarizing beam splitter to detect this one polarizing light component. The second photodetector is arranged on an optical path of the other of the above two polarizing light components separated by the polarizing beam splitter to detect the other polarizing light component. The differential amplifier receives a first signal from the first photodetector and a second signal from the second photodetector. The differential amplifier is electrically connected to each of the first and second photodetectors so as to differentially amplify these first and second signals and output a magnetooptic signal.
.theta.a is set to 45.degree. in the 1/2 wavelength plate since unpolarized noise components have the same amplitude in the first and second photodetectors and are completely cancelled and a signal amplitude represented by the formula (4) is increased.
A Kerr rotational angle of a plane of polarization caused by magnetic Kerr effects of a magnetooptic material is a small angle such as 1.degree. or less. In the above fourth and fifth references, an S/N ratio is increased by optically increasing the Kerr rotational angle by commonly using the above magnetooptic differential detecting method. In this case, after light is reflected on the beam splitter, the Kerr rotational angle is increased by setting reflectivities of P and S polarized lights of the beam splitter to be different from each other in the above actual detecting optical system.
The magnetooptic signal detecting optical system in a magnetooptic disk pickup at present is constructed by assembling many bulky optical elements into each other as explained with respect to the actual detecting optical system. Accordingly, cost of the magnetooptic signal detecting optical system is increased and a strict assembly accuracy is required for the magnetooptic signal detecting optical system. Therefore, a detecting system integrated device is proposed. In this detecting system integrated device, bulky optical elements and photodetectors are functionally integrated on a silicon substrate.
In accordance with the above second literature, three focusing grating couplers of a face separating type are used as this detecting system integrated device. This detecting system integrated device using the three focusing grating couplers is proposed by Nishihara Institute in Osaka University in Japan. In the above third literature, the detecting system integrated device uses a TE/TM mode splitter having a tapered coupling portion and proposed by the inventors of this patent application. However, in each of these literatures, the detecting system integrated device does not use a means for increasing an S/N ratio in the magnetooptic signal detecting optical system in the above magnetooptic disk pickup at present.